The present invention generally relates to optical storage of information and more particularly to a semiconductor optical memory device and system for storing information in the form of optical data.
Various optical data storage systems are currently used for storage of large amount of data, such as optical disks or magneto-optical disks. In such optical data storage systems, it is known that there is a limitation in the recording density of data because of the relatively large wavelength of the optical beam used for the recording.
On the other hand, there is proposed a frequency selective optical data storage system in the U.S. Pat. No. 4,101,976, wherein a multiple recording of information on a common recording medium is achieved by changing the wavelength of the optical beam used for recording. This system employs the so-called photochemical hole burning effect of polymer compounds in which optical absorption of the polymer compound for the light of particular wavelength is saturated after the irradiation of an optical beam having the same wavelength.
FIGS. 1A and 1B show this photochemical hole burning effect conventionally known for the polymer compounds, wherein FIG.1A shows the absorption spectrum before the irradiation of the optical beam and FIG.1B shows the absorption spectrum after the irradiation. As can be seen in FIG.1B, there appear a number of dips or lines in the absorption spectrum in response to the wavelength of the optical beam used for the irradiation. In correspondence to each dip, the absorption coefficient of the polymer is decreased and the optical beam having the wavelength corresponding to the dip passes through the polymer. This phenomenon of photochemical hole burning is attributed to the existence of localized quantum states formed in the chemical bond of the polymer compound. Thus, when an optical beam having the energy that matches the energy level of the quantum state is irradiated, the corresponding transition of carriers is induced in the polymer compound. Once the carriers are excited, the existence of the carriers blocks the transition of other carriers to the same energy level and the polymer compound stops absorbing the optical beam of the same wavelength irradiated thereafter.
In using such a frequency selective multiple recording of optical data for the future optical computers and the like, it is desirable to produce the recording medium by a semiconductor material. With the use of the semiconductor material and the processing techniques thereof, it would be possible to design the structure of the recording medium such that an extremely large capacity of information storage is achieved by using an optical beam of which wavelength is in the convenient range. It should be noted that, by using the currently available semiconductor techniques, it would be possible to provide a large number of memory elements within a region of the focused beam spot of the optical beam that may be typically about 1 .mu.m in diameter. Further, it would be possible to design the band structure such that the semiconductor material interacts with the optical beam of a desired frequency range.